Functionalization of adhesives and analysis of their application requirements

Abstract : Taking binding and packaging, screw locking, general and aerospace structural adhesives as examples, the development process of adhesives from high-performance, functional, high-functionality to intelligent is briefly discussed, and the functionalization and performance of adhesives are preliminarily analyzed. Requirements, performance requirements and application requirements.

Keywords: Adhesives; Functionalization; Performance requirements

Introduction

In recent years, the words “functional adhesives” and “functionalized” have gradually appeared in newspapers and periodicals, as well as albums and monographs. People define "functional adhesives" as "adhesives with special functions" [1].

Adhesive performance requirements are determined by the application requirements. If the stamp is affixed to the envelope, the performance requirement is very low; but the same is the bonding of the paper, and the binding or binding of the paper package will require more. From small to one screw fastening to structural bonding of up to a large aircraft, performance requirements are gradually increasing and increasing.

Adhesives, also known as adhesives, should be adhesive and adhesive in their definition. What are the "special features" of functional adhesives? The functionalization of adhesives is related to their performance requirements [2]. The performance requirements are in turn determined by the application requirements. Then, does the current demand have any macro effects, and what kind of changes in performance requirements will it cause?

The process of adhesive functionalization was investigated, and its correlation with performance requirements and requirements was analyzed. It may have some reference effects on the future research and development of adhesives. This article intends to discuss several application examples.

1 Bookbinding and Paper Packaging Bonding - Reversibility of Bonding

For bonding, in the past, only unidirectionality was generally considered, and the better the adhesion, the better, irrespective of its debonding. In recent years, the reversibility of adhesives has been put on the agenda, such as how large-scale fiber-reinforced plastic bonding products need to be disassembled after they are scrapped. This problem also exists in the binding of books and magazines and the manufacture of packaging machines, because it recycles paper after it is scrapped. Adhesive material hot melt adhesive does not have an adverse effect on this. One of the means of debonding the hot melt adhesive is to decompose it, and there are two methods of decomposition and biodegradation of water and alkali.

(1) Water and alkali decomposition [3]

Hot-melt adhesives can be detacked with hot water or alkali, and must be water-soluble or water-dispersible. There are three main types of hot melt adhesives with this property.

Polyvinyl alcohol (PVA) type: Adhesives with this function must be partially alcoholylated PVA with a high acetic acid content (50%-80%) and a low degree of polymerization (less than 500), and are generally not used alone. Add plasticizers and other additives.

Acrylic type: An acrylic type polymer having a higher carboxyl group can be decomposed or dispersed with an alkali, and the adhesion force can also be improved due to the presence of a carboxyl group.

Polyester type: This type of glue has good moisture resistance and heat resistance, and if it is used as a hydrolyzable glue, a hydrophilic group must be introduced therein, among which there is a carboxyl group, and more preferably a sulfate group. The functionalization of such glues must take into account the balance between water resistance and hydrolyzability.

(2) Biodegradability [4]

Although the amount of glue used in paper packaging containers is small, it has a great influence on the repulping of the packaging. In the papermaking process, some recycled paper such as packaging is sometimes added to the original fiber. The traditional hot melt adhesive does not degrade, and it can cause pinholes and flaws in the newly made paper, affecting printing and aesthetics.

At present, the biodegradable polymers marketed are mostly natural types, and the synthetic types are almost polyester type hot melt polymers, among which polylactic acids are of great interest. So, on the one hand, it can be decomposed by microorganisms; on the other hand, it can also be prepared from renewable resources and lactic acid formed by microbial fermentation. Plants convert CO2 in the air into sugars through photosynthesis, which in turn converts them into lactic acid. The polylactic acid produced is released into the air by microorganisms that become CO2. Taking nature and returning to nature, this CO2 life cycle is an ideal circulation method for sustainable development materials.

The functionalization of such glues also requires attention to the balance of opposite properties.

The balance between brittleness and flexibility. Poly-L-lactide (PLLA) homopolymers can be prepared by the ring-opening polymerization of lactic acid self-polymerization or its dimer lactide. Its glass transition temperature is 55-60°C, its melting point is about 170°C, its tensile strength is 50 MPa, and its elongation at break is 2%. This shows that its melting point is high and it is brittle. If it is used as an adhesive, it needs to be added with a flexible component. Poly(ε-caprolactam) (PCL) has a glass transition temperature of about -60°C, a melting point of about 61°C, a tensile strength of 20 MPa, and an elongation at break of 1000%. Both PLLA and PCL and their copolymers can be degraded in biological environment. Some people have evaluated PLLA/PCL polylactide-based hot melt adhesive with a molar ratio of 81:19. The results indicate that the copolymer is a potential replacement for biodegradable adhesives for packaging. Compared with traditional non-degradable ethylene-vinyl acetate (EVA) type hot melt adhesives, most of them have been tested, such as application time, hardening time, viscosity, weight loss, and tensile strength, lap shear strength and elongation, Both are equivalent.

The balance between decomposition and stability. Untreated PLLA/PCL copolymers have different mechanical properties depending on the geometry of the sample and can be decomposed in a few days, and the polymer is decomposed during processing. The method of enhancing the stability is that the sample is dried prior to processing and processed under N2 atmosphere protection. It is more effective to treat the untreated copolymer with peroxide and acetic anhydride. The decomposability of stabilized hot melt adhesives in food and similar packaging processes is no longer a problem.

(to be continued)